1. Field of the Invention
The present invention relates to light valves used in display devices, and in particular, to integrated circuits using liquid crystal on silicon (LCOS) for creating LCOS display pixels forming high resolution light valves.
2. Description of the Related Art
Liquid crystal displays (LCDs) are becoming increasingly prevalent in high-density projection display devices. These conventional high density projection-type color display devices typically include a light source which emits white light. Dichroic mirrors separate the white light into its corresponding red, green and blue (RGB) bands of light. Each of these colored bands of light is then directed toward a corresponding liquid crystal light valve which, depending upon the image to be projected, either permits or prevents light transmission. Those RGB bands of light which are permitted to be transmitted through the light valves are then combined by dichroic mirrors or a prism. A projection lens then magnifies and projects the image onto a projection screen (Examples of circuit designs and fabrication techniques involving liquid crystal pixel circuits can be found in U.S. Pat. Nos. 5,247,289, 5,461,501 and 5,550,072, the disclosures of which are incorporated herein by reference.)
FIG. 1 illustrates a conventional LCD projection-type imaging system 100. Imaging system 100 includes a light source 101. White light is emitted from light source 101. Once the light hits the prism 103, the light is separated into its red, green and blue colored bands of light by dichroic filter coatings. Colored light is directed toward LCD light valves 105. When reflected off light valve 105, the colored light waves travel back through the prism and through projection lens 107. Lens 107 magnifies and projects the synthesized color image onto projection screen 109.
Conventional LCD light valves are formed by confining a thin layer of liquid crystal material between a top plate and a bottom plate. The top plate is a translucent substrate (typically glass) having one large electrode on a surface adjacent to the liquid crystal material. The bottom plate is generally interconnect overlying a storage capacitor structure formed within a silicon substrate.
FIG. 2 illustrates a cross-sectional view of adjacent pixel cell structures lacking a light absorbing layer, that form a portion of a conventional light valve. Portion 200 of the conventional light valve includes a glass top plate 202 bonded to a interconnect 204 by a sealing member (not shown). The sealing member serves to enclose a display area and to separate glass plate 202 from interconnect 204 by a predetermined minute distance. Thus, the light valve has an inner cavity 206 defined by the glass plate 202 and interconnect 204. Liquid crystal material 211, such as polymer dispersed liquid crystal (PDLC), is sealed in inner cavity 206.
Portion 200 of the conventional light valve depicted in FIG. 2 shows adjacent pixel cells 210a and 210b having reflective pixel electrodes 212a and 212b, respectively. Reflective pixel electrodes 212a and 212b are formed as part of third metalization layer 214 of interconnect 204. The surfaces of adjacent pixel electrodes 212a and 212b are covered with a reflecting layer 216. Reflecting layer 216 serves to reflect away white light incident to the pixel cell as described above in connection with FIG. 1. Adjacent pixel electrodes 212a and 212b are electrically coupled to respective storage capacitor structures 218a and 218b formed in underlying silicon substrate 205.
During operation of pixel cells 210a and 210b, driving circuits (not shown) are electrically coupled with storage capacitors 218a and 218b through row select lines 220a and 220b formed as part of first metalization layer 222 of interconnect 204. Storage capacitors 218a and 218b in turn transmit voltages to pixel cell electrodes 212a 25 and 212b through portions of first, second, and third metalization layers 222, 224, and 214 of interconnect 204.
First metalization layer 222 is electronically isolated from silicon substrate 205 by first intermetal dielectric layer 226. Second metalization layer 224 is electronically isolated from first metalization layer 222 by second intermetal dielectric layer 225. Third metalization layer 214 is electronically isolated from second metalization layer 224 by third intermetal dielectric layer 228.
The selective application of voltage to pixel electrodes 212a and 212b switches pixel cells 210a and 210b of light valve 200 on and off. Specifically, a voltage applied to a pixel electrode varies the direction of orientation of the liquid crystal material on the pixel electrode. A change in the direction of orientation of the liquid crystal material at the pixel electrode changes the optical characteristics of the light traveling through the liquid crystal. If the light valve contains twisted nematic crystal, light passes through the light valve unchanged where no voltage is applied to the pixel electrode, and the light is polarized if a voltage is applied to the pixel electrode. If the light valve contains PDLC, light passes through the light valve unchanged where a voltage is applied to the pixel electrode, and light is scattered if no voltage is applied to the pixel electrode.
However, as the densities of such displays have increased, limitations have been encountered. For example, one limitation on this density is the need for a minimum amount of storage capacitance in each pixel. For example, depending upon the frame rate of the display, the decay of the charge on the storage capacitor can lead to instability of the level of grey scale illumination, thereby leading to degradation in the image quality.
Accordingly, it would be desirable to have a technique for increasing individual pixel storage capacitance while maintaining the desired pixel density within the pixel array.
A liquid crystal pixel circuit in accordance with the present invention provides increased storage capacitance by using multiple storage capacitors with minimal, if any, increase in required circuit area. According to one embodiment, two storage capacitors are integrated in a vertical, or stacked, configuration in which a common, or shared, electrode is used for both capacitors.
In accordance with one embodiment of the present invention, an integrated liquid crystal pixel circuit with a plurality of storage capacitors includes an integrated circuit which includes a semiconductor substrate, a charge terminal, a reference terminal, a pass transistor, a liquid crystal cell and a plurality of storage capacitors. The charge terminal is within the integrated circuit and configured to receive and convey an electrical charge. The reference terminal is within the integrated circuit and configured to receive and convey a reference voltage. The pass transistor is within the integrated circuit, coupled to the charge terminal and configured to receive a control signal and in accordance therewith conduct the electrical charge. The liquid crystal cell is atop the integrated circuit, coupled to the pass transistor and responsive to the electrical charge. The plurality of storage capacitors is within the integrated circuit, coupled in parallel between the pass transistor and the reference terminal and configured to receive and store the electrical charge.
In accordance with another embodiment of the present invention, an integrated liquid crystal pixel circuit with a plurality of storage capacitors includes an integrated circuit which includes a semiconductor substrate, a charge terminal, a reference terminal, a transistor, a liquid crystal cell and first and second capacitors. The charge terminal is within the integrated circuit and configured to receive and convey an electrical charge. The reference terminal is within the integrated circuit and configured to receive and convey a reference voltage. The transistor is within the integrated circuit, coupled to the charge terminal and configured to receive a control signal and in accordance therewith conduct the electrical charge. The liquid crystal cell is atop the integrated circuit, coupled to the pass transistor and responsive to the electrical charge. The first capacitor is within the integrated circuit, coupled between the pass transistor and the reference terminal and configured to receive and store a portion of the electrical charge. This capacitor includes a first electrode, a shared electrode and a first dielectric material between the first and shared electrodes. The second capacitor is within the integrated circuit, coupled between the pass transistor and the reference terminal and configured to receive and store another portion of the electrical charge. This capacitor includes the shared electrode, a second electrode and a second dielectric material between the second and shared electrodes.
In accordance with still another embodiment of the present invention, a method of fabricating an integrated liquid crystal pixel circuit with a plurality of storage capacitors includes the steps of:
providing a semiconductor substrate with an upper surface;
forming a transistor within the semiconductor substrate;
forming an isolated doped region within the semiconductor substrate below the upper surface;
forming a first conductive layer over the upper surface, wherein the first conductive layer is insulated from the isolated doped region;
forming a first insulating layer over the first conductive layer;
forming a second conductive layer over the first insulating layer, wherein the second conductive layer is insulated from the first conductive layer;
forming a second insulating layer over the second conductive layer;
forming a third conductive layer over the second insulating layer, wherein the third conductive layer is insulated from the second conductive layer;
forming a conduction path between the isolated doped region and the second conductive layer, wherein
the mutually insulated isolated doped region and first conductive layer together form a first capacitor,
the first conductive layer, the first insulating layer and the second conductive layer together form a second capacitor, and
the conduction path causes the first and second capacitors to be coupled in parallel;
forming a liquid crystal cell; and
conductively coupling the liquid crystal cell, the transistor and the first and second capacitors.